Membrane separation using a filtration membrane is employed in various fields as a substance separation method which is simple and consumes only a small amount of energy. The working principle of the membrane separation is such that) basically, substances are screened by filtration, depending on the size of pores present In the membrane. Therefore, a uniform desired pore size is important for performance characteristics of the membrane.
On the other hand, defects are produced in the filtration membrane in some cases during production or use of the filtration membrane. The most typical defect is a defect called pinholes. The pinholes are a small number of pores having a size larger than the desired pore size of the filtration membrane. If the pinholes are present, substances capable of passing through the pinholes are not screened, so that a desired substance in the filtrate is contaminated with the substances which should be essentially excluded. Thus, the separation efficiency is lowered.
As a method for detecting the pinholes, the minute defect of the filtration membrane, it is known to use a method which utilizes the surface tension of a liquid which comprises pressurizing with a gas one of two spaces formed by partition with the filtration membrane, filling the other with the liquid, applying such a pressure that the gas flows out through the pinholes but not through normal pores, measuring the flow rate of gas leaking from the pinhole portions, and thereby investigating the presence of the pinholes.
For example, JP-A-1-307409 discloses three measuring methods, i.e., a method of directly measuring the flow rate of a gas on the supply side (the starting solution side) after a definite time after the start of pressurization, a method of measuring the flow rate of a liquid pushed out by the gas, on hollow yarn side (the filtrate side), and a method of measuring the dropping of the liquid level on the hollow yarn side (the filtrate side)
In addition, JP-A-60-94105 discloses two measuring methods, i.e., a method of measuring a pressure change due to the leakage of a gas on the basis of a pressure decrease on the starting solution side, and a method of directly measuring the leakage of the gas (corresponding to the amount of the gas supplied) by means of a gas flowmeter attached on the starting solution side.
Further, JP-A-5-157654 discloses a method comprising providing an air-tight gas chamber on the filtrate side and measuring the leakage of the gas on the basis of a pressure increase on the side on which the gas leakage and inflow occurs.
However, when the pinholes are small in size or number, the degree of gas leakage is low, so that it is often difficult to directly measure a low gas flow rate with high precision using a measuring instrument in the direct measurement method for measuring the gas flow rate.
The method of measuring the amount of a liquid pushed out by the gas and the method of measuring a pressure change by providing a gas chamber on the liquid enclosure side permit indirect and high-precision measurement of a low gas flow rate but involve problems. For example, when the membrane expands remarkably due to pressurization or when a precise measurement is required in which the expansion of the membrane cannot be neglected: the precision of measurement is decreased because a liquid outflow or a pressure change, or both are caused by the expansion of the membrane simultaneously with those caused by gas leakage, and the outflow and/or pressure change caused by the expansion are usually undistinguishable from those caused by the gas leakage.